The present disclosure relates generally to well logging with neutron-induced gamma-rays and, more particularly, to well logging with neutron-induced activation gamma-rays.
Using nuclear downhole tools, the elemental composition of a subterranean formation may be determined in a variety of ways. An indirect determination of formation lithology may be obtained using information from density and photoelectric effect (PEF) measurements from gamma-ray scattering in the formation. A direct detection of formation elements may be obtained by detecting neutron-induced gamma-rays. Neutron-induced gamma-rays may be created when a neutron source emits neutrons into a formation, which may interact with formation elements through inelastic scattering, high-energy nuclear reactions, or neutron capture.
As a result of inelastic or capture reactions, certain formation nuclei may become radioactive. Each radioactive isotope in the formation may have a characteristic half-life and a characteristic decay path to a non-radioactive element. The decay of most radioactive elements may be accompanied by the emission of one or more characteristic gamma-rays. These characteristic gamma-rays may be used to identify the element of the formation that is decaying, and thus may indicate a unique formation element that has been activated by inelastic scattering or neutron capture.
Various formation measurements may be obtained based on the above-described nuclear reactions. For example, fracture height determination in a formation may be undertaken by injecting radioactive tracer elements into a formation with fracture fluid and proppant, subsequently measuring characteristic gamma-rays emitted by the tracer. However, the use of a radioactive tracer may introduce a number of regulatory, environmental, and other challenges, as the radioactive tracer may be in liquid form and thus easily dispersible. As such, certain techniques have been developed to avoid the use of radioactive tracer in fracture height determination. These techniques may involve the injection of an inert liquid tracer into the formation, which may be subsequently bombarded with neutron radiation to activate the tracer in the liquid. In carrying out these techniques, however, the source of the activating neutron radiation may be moved away from the point of measurement, and the activation radiation may be measured at a later time when a gamma-ray detector or other detector passes by this point. In certain cases, the intervening time between activation and measurement may allow materials in the tracer-containing fracture fluid to move, which may result in an incorrect interpretation of a formation fracture or other formation properties.